1. Field of the Invention
The invention relates to a projection objective for micro-lithographic projection exposure apparatus, such as those used for the production of microstructured components.
2. Description of Related Art
Microlithographic projection exposure apparatus, such as those used for the production of large-scale integrated electrical circuits, have an illumination device which is used to generate a projection light beam. The projection light beam is aimed at a reticle which contains structures to be imaged by the projection exposure system, and which is arranged in an object plane of a projection objective. The projection objective forms a reduced image of the structures of the reticle on a photosensitive surface, which is located an image plane of the projection objective and may for example be applied on a wafer.
Owing to the small size of the structures to be imaged, stringent requirements are placed on the imaging properties of the projection objective. Imaging errors can therefore be tolerated only to a very small extent.
In general, imaging errors which occur are assigned to the following categories. On the one hand, there are imaging errors which result from the design of the projection objective, i.e. in particular the specification of dimensions, materials and distances in the optical elements contained in the projection objective. These design errors will not be considered below.
On the other hand, there are imaging errors which are attributable to production or material errors and which it is generally sensible to correct only once the objective is finished. Examples of production errors include so-called form errors, which are intended to mean deviations from surface accuracy in the case of optical surfaces. Material errors, however, do not generally affect the condition of the optically active surfaces, i.e. ones through which projection light passes, but lead to inhomogeneous refractive index profiles inside the optical element. The possible causes of such imaging errors due to production or material will more generally be referred to below as perturbations, which may be locally very limited but which may even extend over a sizeable region of the optical element in question.
In order to correct the wavefront deformations caused by such perturbations, it is known to apply corrective structures to suitable optical surfaces of the projection objective by means of reprocessing, which generally involves material erosion. The reprocessing gives the surface an aspherical shape that is generally not rotationally symmetric, and which differs from the shape on which the design of the projection objective was based. Such reprocessing methods are described at length in an article by C. Hofmann et al. entitled “Nanometer Asphären: Wie herstellen und wofür?”, Feinwerktechnik und Meβtechnik 99 (1991), 10, pp. 437 to 440.
The way in which the corrective structures required for the compensation may be deduced from the wavefront deformations, which can generally be recorded by measuring techniques, is described at length in U.S. Pat. No. 6,268,903 B1. In the method described there, the optical element whose surface is to be locally reprocessed for perturbation compensation is preferably a plane-parallel plate, which is arranged between the reticle and the projection objective of the projection exposure apparatus. The known corrective element may furthermore be arranged inside the objective, before or after a shutter contained therein, in which case a position where the projection light beam has a particularly small cross section is preferred.
However, it has been found that not all optically active surfaces inside the projection objective are equally suitable for correcting a wavefront deformation by local reprocessing. Since projection objectives generally contain a large number of optically active surfaces, it is not in fact readily possible to determine the correction potential computationally for all these surfaces. As a rule, therefore, empirical values or rules of thumb are relied upon when choosing which optically active surfaces should be reprocessed in order to compensate for perturbations. For example, a small diameter of the projection light beam is highlighted as an essential criterion in the method known from the aforementioned U.S. Pat. No. 6,268,903 B1.
It has been found, however, that the imaging properties often cannot be improved sufficiently with the known criteria for the selection of surfaces to be reprocessed.